1. Field of the Invention
The present invention relates to an apparatus for reading images of various subjects, for example, fingerprints, documents, patterns, etc.
2. Description of the Related Art
Fingerprints have long been used as means for identifying individuals, and a great amount of research and development on automated fingerprint identification systems is being made. As society becomes more and more information-oriented, there is an increasing demand particularly for security in respect of the identifications of individuals who attempt to enter or exit important administration areas, such as computer rooms, or to access various information terminals and banking terminals.
A variety of fingerprint readers used for the above kind of identifications of individuals, which instantaneously detect high-contrast images formed in accordance with the irregularities of the fingerprint patterns, have been developed.
As an example of the fingerprint readers, the one shown in FIG. 32, which is operable according to the total reflection method, is known. This fingerprint reader is constructed of a light-emitting unit 10, a lens 11, a rectangular prism 12, and a CCD camera 13. In operation, total reflection occurs to a beam emitted from the light-emitting unit 10 on the slanted surface of the rectangular prism 12, and the light reflected from the prism 12 is then received by the CCD camera 13. When a finger 14 is placed on the slanted surface of the prism 12, total reflection is caused on the slanted surface corresponding to depressions 15 of the fingerprint, which are not in contact with the slanted surface, and incident on the CCD camera 13, while irregular reflection rather than total reflection occurs on ridges of the fingerprint in contact with the slanted surface of the prism 12 due to a disparity in the refractive index between the finger 14 and air. Thus, the contrast of the totally reflected light and the scattered light is detected by the CCD camera 13, thereby obtaining a high-contrast fingerprint image.
In addition to the above total-reflection method, the optical-path separation method and the photoconductive method using a glass conductive plate are known.
However, the above-described methods require a rectangular prism and a lens, thereby making it difficult to reduce the size of the overall fingerprint reader. What is worse, these prism and lens are expensive, thus increasing the cost. Further, scattered light from the ridges of a fingerprint, as well as totally reflected light from the depressions 15, inconveniently impinges on the CCD camera 13, which makes it hard to clearly distinguish the ridges from the depressions 15 of the fingerprint, resulting in a low-contrast image.
To overcome the aforedescribed problems inherent in a conventional reader, a fingerprint reader using a fiber optic plate (FOP) 19 formed by a bundle of a numerous optic fibers, as illustrated in FIG. 33, has been developed. This fingerprint reader is constructed of a light-emitting unit 18 formed of a light emitting diode, a fiber optic plate 19, and a CCD 20 intimately contacting the output surface of the FOP 19. In this reader, as illustrated in FIG. 34, total reflection occurs to the light emitted from the light-emitting unit 18, and a light pattern representing a fingerprint image is formed according to the irregularities of the fingerprint pattern. The light pattern is then directly transmitted to the CCD 20 via the FOP 19.
Since a lens is not employed for the above fingerprint reader using a fiber optic plate, a space originally required for image formation can be saved, thereby enhancing the downsizing of the image reader. On the other hand, a light-emitting unit, such as a light-emitting diode, should be disposed in the vicinity of the input surface of the fiber optic plate 19. This does not necessarily satisfy the requirements of reducing the size, in particular, the thickness, of the fingerprint reader. Additionally, this type of reader is costly.